This invention relates to an improved apparatus for greater utilization of the heat produced by burning fuel within a furnace of the type commonly employed for heating domestic homes and the like. More particularly, the present invention provides that a flue pipe with heat exchange surfaces extends through the hot air plenum and through a cold air supply duct for extracting heat from flue gases before the gases are discharged into a chimney and, in combination therewith, a heat sensor to deliver a signal to turn ON a driven blower in response to the air temperature in the hot air plenum.
Family dwellings are commonly heated by forced air heating systems including a furnace wherein fuel such as oil, liquid gas or natural gas is burned within an enclosed combustion chamber which is, in turn, located within a stream of forced air to heat the air. The heated air is then conducted by a plenum chamber connected to a hot air manifold to distribute the hot air to various places in the dwelling. The burnt gases in the combustion chamber are drawn through a flue pipe into a chimney. Substantial quantities of heat are lost in the chimney with the rising currents of heated air along with the burnt gases. When natural gas is burned in the combustion chamber of a furnace, the temperature at the end of the flue pipe in the chimney is about 450.degree. F. When oil is burned in the combustion chamber, the temperature at the discharge end of the flue pipe in the chimney is about 800.degree. F. Since the heated air in the chimney is exhausted directly to the outside environment, there is a substantial loss of heat produced by the furnace. This lowers the efficiency of the furnace, increases the heating cost and wastes fuel. These represent serious disadvantages because of the ever-increasing cost of fuel and the decreasing fuel supplies.
The present invention greatly increases the efficiency of heating systems, particularly those commonly employed in family dwellings by utilizing residual heat in the flue pipes in the forced air system to such an extent that the temperature of the burnt gases entering the chimney is greatly reduced.